phase space (thing)

A little more practcally, a phase space is any imaginary space, plotting measurements of a few phemonena pertinent to a particular physical or mathematical system.

If you were showing the states of a pendulum, for example, you might choose a two-dimensional phase space so that you could plot the pendulum's horizontal position versus its speed at any one given point in time.

You might even throw time in to your phase space to get three dimensions (although time with one other dimension is usually called a time series rather than a phase space), but you would definitely *not* add a dimension to plot the pendulum's vertical position, since that doesn't give you any new information.